Propylene/ethylene copolymers are already known in the art for the production of pipes.
For example, according to international patent application WO 97/33117, one can obtain pipes of the polypropylene plastic material having high creep resistance, high long-term pressure resistance, improved stiffness, and resistance to rapid crack propagation. According to the document, the catastrophic failure of a pipe of polypropylene plastic is prevented when the pipe is made of several layers of different polypropylene plastic material, wherein at least one layer consists of a broad molecular weight distribution (MWD) polypropylene that provides the high creep resistance and at least one layer consists of an elastomer-modified polypropylene that improves the impact strength. The broad MWD polypropylene is a mixture of a very high molecular weight propylene random copolymer with 1-10 wt % of ethylene or a higher-α-olefin repeating units and of a low molecular weight propylene polymer with low (up to 1 wt %) or zero comonomer.
When small diameter pipes are needed it is beneficial to have limited wall thickness of the pipe. The thinner wall reduces the amount of material in the pipe and improves the efficiency of the pipe manufacturing in terms of feed rate. However, when the wall thickness becomes small, the pipe may become brittle. Thus, it is necessary to use a material having high impact resistance, especially at low temperature.
Pipes for radiant heating may be made by cross-linked polyethylene. The cross-linking helps to avoid fatigue of the polymer material under the stress of water pressure and temperature cycling. However, with the cross-linking, the polyethylene pipe decreases in flexibility.
Moreover, the pipe typically arrives at the site in rolls, and its “memory” of the rolled shape complicates the task of securing it in straight runs.
The applicant found that small amount of heterophasic copolymers having certain features can be added to propylene/ethylene copolymer in order to improve impact properties and flexibility of small diameter pipes.